This invention relates to a polymer thick film (PTF) composition containing a metal catalyst and graphite, which can be used to print sensing/working electrodes for electrochemical biosensors based on hydrogen peroxide detection. Electrochemical biosensors which are combinations of an electrochemical sensor and a biomolecule recognition element are useful in the analyses of biological analytes such as glucose, cholesterol, creatinine, alcohol, uric acid and lactate in body fluid, and are therefore useful in the field of medical devices and analytical instruments for medical diagnostics.
The majority of amperometric electrochemical biosensors fall within two categories: (1) metal-catalyzed electrochemical biosensors or (2) electron-transfer mediator modified electrochemical biosensors. For example, a metal-catalyzed glucose sensor detects the hydrogen peroxide by-product which is produced in a one-to-one ratio from glucose through an enzyme-catalyzed air oxidation process, such as:
Glucose+O2glucose oxidase (GOD) greater than glucolactone+H2O2
U.S. Pat. No. 3,539,455 (1970) by Clark discloses a platinum based glucose sensor useful for self-monitoring of glucose by diabetics. Guilbault and Lubrano (1973) reported amperometric biosensors having an immobilized-enzyme platinum electrodes suitable for glucose sensor applications. Mizutani et al. (1992) reported a platinum/carbon paste (CP) composition with a 1/9 Pt/C ratio suitable for making glucose sensors. U.S. Pat. No. 4,970,145 (1990) to Bennetto et al. discloses a biosensor with a porous enzyme electrode comprising platinized carbon paper having a fluoropolymer binder. These platinum/C based biosensors have sensitivity for detection of glucose only at concentrations of millimolar (mM) glucose with an electric current response of  less than 20 uA/cm2 mM glucose. Furthermore, the use of a metal or metal/carbon with a high metal content as the working electrode leads to high material costs and has a drawback of loosing hydrogen peroxide due to the metal-catalyzed decomposition of hydrogen peroxide.
An electron-transfer mediator modified electrochemical biosensor relies on a fast electron-transfer mediator, which is typically an outer-sphere metal complex or organic compound, to assist the shuttling of electrons from reduced enzyme molecules to the working electrode. The reduced enzyme results from an enzyme molecule receiving two electrons from a glucose molecule, such as:
Glucose+(GOD)oxidizedxe2x86x92Glucolactone+(GOD)reduced
Electrochemical biosensors having carbon based working electrodes modified with electron-transfer mediators have been reviewed by Wring and Hart (1992). Biosensors having working electrodes modified with redox polymers have been reviewed by Pishko (1995). These mediator modified biosensors are capable of measuring glucose at mM concentration with an electric current response of  less than 20 uA/cm2.mM glucose. There remains a need for materials suitable for fabrication of enzyme electrodes with high catalytic activity/current response and low background current noise to expand the capability of biosensors for monitoring biological analytes at the micro-molar (uM) level and to assure a high confidence of detecting low level of analystes in body fluids. One example was given by Tamada, Bohannon and Potts (1995) who reported the iontophoretic extraction of body fluid. The body fluid can then be analyzed in situ for a glucose levels and thus provide a method for non-invasive monitoring of glucose. The glucose concentration in the extracted body fluid is typically in the micro-molar level which produced electric current in the nano-ampere (nA) level, and thus requires a biosensor with low detection limit of glucose determination. A key limiting factor which affects the glucose detection limit is electrochemical signal noise or background current, which may be from electrochemically active impurities or temperature fluctuation etc. It is desirable that a biosensor has low background current which also does not greatly vary with temperature fluctuation.
Furthermore, it is desirable that catalyst materials for the working electrode be low cost and suitable for low cost fabrication of disposable biosensors by conventional printing processes. It is the object of the invention to overcome the deficiencies of current carbon-based materials for working electrodes.
The invention is directed to a polymer thick film conductor composition comprising, based on solids: (a) 0.1-5% wt. conductive metal particles selected from the group consisting of finely divided powders of platinum group metals and mixtures; (b) 62-85% wt. of graphite conductive filler; and (c) 14-35% wt. of a thermoplastic polymer.
The invention is still further directed to a conductor composition for use in electrochemical sensor applications comprising, based on solids: metallized particles comprising platinum group metal particles and mixtures thereof; deposited on support particles selected from graphite, surface modified graphite and mixtures thereof; with the provisos wherein the platinum group metal particles and mixtures thereof are in the range of 0.1-5% wt. and wherein the support particles and any optional graphite conductive filler are in the range of 62-85% wt.; and 14-35% wt. thermoplastic polymer.
The PTF compositions covered in the present invention are intended for use in printing sensing or working electrodes in electrochemical biosensor applications, and more specifically, amperometric glucose sensors. The PTF composition offers glucose sensors with enzyme electrodes extremely high sensitivity for analysis of glucose at micro-molar levels, for example, a method for non-invasive monitoring of analytes transdermally extracted from the body.
The PTF conductive composition is designed to have the following characteristics when used as a working electrode:
(a) Printed PTF working electrodes have high electrocatalytic activity toward a target chemical or biomolecule, and therefore are able to provide strong electrical signal even at extremely low concentrations of the target chemical. Furthermore, the PTF working electrode should not introduce background current signal noise which would limit the ability of a biosensor to detect low concentrations of a target chemical.
(b) The PTF conductive composition is designed to have stable and consistent electrocatalytic activity enabling the biosensor to handle multiple analyses for continual glucose monitoring.
c) The PTF composition is intended for use in manufacturing of disposable biosensors and it is designed to be low cost.
d) The PTF composition has good rheological properties to facilitate manufacturing of sensors by conventional printing processes.
Through a unique combination of platinum group metal electrocatalysts and graphite materials, a low cost electrocatalyst system containing as low as 0.5/99.5 of metal/graphite ratio produces a biosensor working electrode to detect glucose levels of 100 nano-molar. This unique electrocatalyst in combination with a thermoplastic resin solution provides a low cost and reliable graphite-based PTF conductive composition for use in making disposable biosensors.
In this invention, the PTF conductive composition comprises the following components: (A) metal electrocatalyst selected from the platinum group which converts a chemical signal to an electrical signal (B) a conductive filler particles that provide electrical conductive pathways for the electrical signal, (C) a polymer that serves as the matrix resin holding (A) and (B) in a coating adhering to a plastic film substrate, and (D) an organic vehicle when mixed with the other ingredients make a composition for easy processing and printing of sensors.
A. Electrocatalyst
Electrocatalysts may be utilized in the present invention in two forms: (1) platinum group metal powders or (2) platinum group metals deposited on electrically conductive supports.
(1) Platinum Group Metal Powders
Precious metals are well known for their catalytic activity for organic and inorganic reactions as well as catalytic electrochemical reactions. The metals for use in the invention are metals from the platinum group which are platinum, palladium, rhodium, iridium, ruthenium, osmium and mixtures thereof. Platinum is particularly preferred for use as a electrocatalyst in working electrodes.
For use in a PTF composition, the metal particles are finely divided powder form. Furthermore, a metal powder with high surface area and very small particle size is preferred. Such a metal powder not only reduces metal cost but also provides superior catalytic activity. Metal powder catalysts, such as platinum black, typically has a surface area of  greater than 5 m2/g and is suitable for the metal-graphite compositions covered in this invention. Metal powder catalysts with very high surface area ( greater than 65 m2/g) lead to high sensor background current. Fuel cell grade platinum black powders having a typical surface area of 25-60 m2/g, commercially available from Colonial Metals Inc., Elkton, Md. and Alfa Aesar, Ward Hill, Mass., are particularly suitable. Precious metal powders with surface areas of  less than 5 m2/g, commonly used in thick film compositions for electronics applications, do not have sufficient catalytic activity for sensor applications. Therefore, surface areas  greater than 5 m2/g are preferred.
(2) Platinum Group Metals Deposited on Electrically Conductive Supports (Metallized Graphite)
A metallized graphite particle is platinum group precious metal particles and mixtures thereof directly attached to support particles. Supported metals offer potential benefits of high electrocatalytic activity through direct electron-transfer from catalytic sites to the conductive network. The most widely used and preferred catalyst support for use as electrocatalysts is graphite, which is commonly used in carbon electrodes but a modified graphite may also be used. Modification of graphite would be by the same process as described hereinbelow for the modified metallized graphite particles. Graphite and modified graphite particles offer not only good electrical conductivity but also low electrochemical signal interference because of its inertness to electrochemical reaction. Metallized graphite particles suitable for use as the electrocatalyst has about 0.5-10% metal based on the weight of the particle. Higher metal loading not only make sensors unacceptably costly for disposable sensor applications but also leads to background current noise due to high activity in catalyzed side reactions. To achieve high catalytic activity and low material cost, it is also desirable that catalyst particles be deposited in micro-crystalline size, typically  less than 10 nm. Metallized graphite particles can be prepared using a method described in the U.S. Pat. No. 4,044,193. Metallized graphite can be modified to form a modified metallized graphite which enriches the surface functional groups on the graphite which interact with metal catalyst particles. Surface modification processes effective for graphite materials are reduction with strong reducing agents and plasma etching processes. Strong reducing agents such as sodium hypophosphite, sodium borohydride, sodium bisulfite and sodium formate are effective in producing modified metallized graphite materials that show improved sensor performance.
The amount of metal in finely divided metal powder and/or in metallized graphite (referred to collectively as xe2x80x9cmetalxe2x80x9d) in the composition based on solids is about 0.1-5% wt. with a preferred range of 0.3-3% wt. and more preferred of 1-2.5% wt. The preferred metal/graphite ratio is in the range of 5/95 to 0.5/99.5, wherein the graphite constitutes the total amount of graphite found in the composition whether found as graphite supports or graphite conductive filler as described in part B. hereinbelow.
B. Graphite Conductive Filler
Several types of graphite conductive filler may be used in the invention and are described herein. Graphite is commonly used as the conductive filler forming the conductive network in the carbon composition electrodes. Graphite materials suitable for sensor working electrodes may be synthetic; pyrolytic, or natural graphite. Synthetic graphite made from petroleum coke that have good balance of low metal impurities (usually  less than 500 ppm metal contamination) and rich surface functional groups are preferred. Pyrolytic graphite made from natural gas (such as UCP-2 graphite from Carbone of America, Bay City, Mich.) and purified graphites which are purified by a high temperature electro-crystallization process (such as 8315 graphite from Asbury Graphite, Asbury, N.J.) tend to have less surface functional groups for interacting with metal catalysts. Generally, platinum carbon electrodes with pyrolytic or purified graphites tend to have poor sensor performance. Also, natural graphite tends to have poor purity which may lead to high background current noise and potential risk of catalyst poisoning.
Therefore, preferred graphite materials typically have particles with diameters of about 1-30 microns with mean particle diameter in the range of 2-10 micron. Larger graphite particles tend to cause problems in screen printing. Finer graphite particles which have high surface areas tend to raise the background current. Graphite materials can also be modified by the same method as the modified metallized graphite described above.
If metal powder is used in the composition then the amount of graphite, modified graphite powder or mixtures thereof added to the composition based on solids is in the range of 62-85% by weight. When using metallized graphite powder, the amount of graphite found in the composition constitutes the total amount of graphite found in the composition whether found as graphite supports or graphite conductive filler as described herein and it is in a range of 62-85% by weight based on solids in the composition. In order to achieve good electrical conductivity the volume ratio of graphite/binder is maintained in the range of 75/25 to 45/55.
C. Polymer Binder
Thermoplastics are preferred polymeric binders in the composition. Unlike PTF compositions with thermoset binders which require long curing time at high temperature, thermoplastic based PTF compositions can be used in a quick printing-drying process suitable for reel-to-reel sensor fabrication. Suitable thermoplastic binders provide a matrix that holds the electrocatalyst and graphite particles together and forms a coating with good scratch resistance and good adhesion to plastic film substrates. Thermoplastic resins for use in the invention have a glass transition temperature  greater than 40 C. and are selected from the group of acrylic, polyester, vinyl resins, polyimide, and polycarbonate polymers. It is important that the thermoplastic polymers contains no electrochemically active impurities which contribute to background current noise. Polymers which contain aromatic groups on the polymer chain or side chains are preferred. Examples are styrene-containing acrylic copolymers poly(styrene-acrylonitrile) (such as Tyril resins from Dow Chemicals, Midland, Mich.), benzyl methacrylate acrylic copolymer, poly(butadiene-acrylonitrile-styrene), poly(styrene), poly(hydroxyether) (such as UCAR phenoxy resins from Phenoxy Specialties, Rock Hill, S.C.), copolyester resins with tera-, iso- or phthalate aromatic groups (such as Vitel resins from Goodrich, Arkon, Ohio), polycarbonate (such as Lexan resin from General Electric, Pittsfield, Mass.) and polyimide (such as Ultem resins from General Electric). The aromatic groups on the polymers enhance the wetting of the polymers on graphite surfaces, and thus reduce printing defects such as pin-holes caused by polymer de-wetting during printing and drying. The polymeric binders can be dissolved in solvents, or solvent blends to provide a vehicle for making metal-graphite compositions suitable for screen printing.
In the dry electrode coating, the amount of binder is in the range of 14-35% by weight. A lower binder level results in a porous coating which has low scratch resistance and high background current. A higher binder level leads to low electrical conductivity and low electrocatalytic activity, and thus unacceptable sensor performance.
D. Surfactant
The precious metal powder is further dispersed to form a colloidal dispersion by milling the powder in a polymer solution, and a surfactant is added to help stabilize the dispersion catalyst particles. Surfactant molecules tend to migrate to the air surface during drying of the print and thus provide a way to modify the hydrophobicity of Pt/C electrode surface. A surfactant with hydrophilic end groups, such as ethylene oxide units, can improve the wetting of the electrode by water and also enhances sensor performance.
Surfactant may be found in the present invention at 0.01-3% wt. based on solids. Surfactants suitable for the platinum black dispersion are from the group of long-chain fatty acid or their sodium salt, nonionic surfactants such as long-chain fatty alcohol, non-ionic surfactants based on poly(ethylene oxide)-modified fatty acid, fatty alcohol and alkylphenol (such as surfactants under the trade names of Triton(copyright), Tergitol(copyright) from Union Carbide, CT).
E. Solvent
A solvent suitable for the composition should meet the following criteria: a) be able to dissolve the polymer binder; b) have low electrochemical activity and minimal electrochemically active impurities so that very low background current can be achieved; c) have a moderate evaporation rate which quickens drying during sensors printing and production; d) be inert to Pt-catalyzed chemical reactions, such as auto-ignition which can cause safety problem during manufacturing and processing. Most hydroxyl-containing organic solvents, such as alcohol or glycol alkyl ether do not meet requirements (b) and (d), and are not suitable for the PTF composition. Solvents, such as dibasic esters which contain electrochemically active alcohol impurities also are not suitable. Solvents from the groups of alkyl and aryl ketones, aromatic hydrocarbon, glycol diacetates and glycol acetates or mixtures thereof were found to be suitable solvents producing compositions with low sensor background current. Aromatic hydrocarbon solvents which tends to have good wetting on graphite surfaces provides an additional benefit in imparting a slightly shear thinning rheology on the composition and thus improves the printing of graphite based electrodes.
A typical metal-graphite composition may be prepared as follows: (1) a binder solution is prepared by dissolving a thermoplastic polymer in a suitable solvent; (2) a dispersion of metal powder in the binder solution is then prepared by roll-milling; (3) a mixture of binder solution, metal powder dispersion, graphite and/or metallized graphite and solvent are mixed by high speed dispersion methods to make the PTF composition. The resultant composition typically has a viscosity in the range of 10-100 Pa.S and a % solids of 25-40% suitable for screen printing.
F. Sensor Fabrication and Testing Criteria
The electrochemical sensor used for testing sensor performance of the composition of the present invention is based on a three-electrode design containing a working electrode, a counter electrode and a reference electrode. The working electrode is about a 1 cm2 disk of the metal-graphite composition with 10-30 xcexcm thickness. Both the counter electrode and the reference electrode are prints of a silver/silver chloride PTF composition as found in recently allowed U.S. Ser. No. 08/921,183. The sensor is printed on a 5 mil polyester film substrate using a conventional screen printing process. Typically, the printing is done in a multiple print-dry sequence which lays down conductive patterns of electrical contacts for measuring equipment attachments. Dielectric coatings are applied for protecting conductive lines. Sensor testing was done using a custom-made potentiostat.
Three key tests were performed: (TEST A) electrochemical sensor response to hydrogen peroxide and (TEST B) biosensor response to glucose and (TEST C) effect of temperature on biosensor. TEST A is done in a specially designed test cell which holds a 10 mil thick layer of test solution on top of the three-electrodes of the sensor. The cell is filled with a phosphate buffered saline solution (PBS) having pH of 7.5, 0.1 M phosphate and 77 mM NaCl, and the background current is recorded as the steady-state current measured between the working electrode and the counter electrode after a current bias with the potential of the working electrode set against the reference electrode at 0.75V for 10 minutes and at 0.4V for 50 minutes. Then the current response of the sensor after a H2O2 solution is injected in the test cell is recorded vs time. A series of current measurements are made with different H2O2 concentrations. The slope of a linear plot of the current measured at a fixed time point, such as at 60 seconds, against the H2O2 concentration provides a measure of the sensor sensitivity to H2O2. An sensor sensitivity of 20-70 nA/cm2.uM of H2O2 with background current noise in the range of 2-20 nA is acceptable. The preferred range is a sensor sensitivity of  greater than 50 nA/cm2 xcexcM of H2O2 with a background current noise of  less than 5 nA.
TEST B is done on a biosensor which has an enzyme-containing gel disk placed on top of the three electrodes of an electrochemical sensor. The gel disk contains a poly(ethylene oxide) gel, NaCl and phosphate buffer, and glucose oxidase enzyme for glucose oxidation and hydrogen peroxide generation. The biosensor is pre-conditioned with the working electrode on a 0.77V bias for 10 minutes followed by a 0.42V bias for 50 minutes, and the steady-state current measured at the end is the background current. Then 10 micro-liter volume of a 0.2 mM glucose solution sample is added to the gel. The current response is recorded verses time. The charge/electron response at time (t) from the glucose sample can be calculated by integrating over time (t) the current response minus the background current. The charge response at time (t) divided by the theoretical total charge generated from the glucose sample gives the % charge response (% recovery) recovered by the biosensor at time 2.5 minute provides a measure of the sensitivity of the biosensor to glucose. Glucose recovery  greater than 20% at 2.5 minute recovery time were acceptable results, but more preferred is  greater than 30% at 2.5 minute recovery time.
TEST C is done on a biosensor that has an enzyme-containing gel disk placed on top of the three electrodes of an electrochemical sensor, as in TEST B. The biosensor is first held at 32xc2x0 C., and the working electrode is preconditioned for 10 minutes at 0.77V bias followed by 50 minutes at 0.42V bias. The background current is then measured every fifteen minutes over the next 7 hours. During each 15 minute cycle, the 0.42V bias is first turned off for 5 minutes and then turned on for 10 minutes. The background current is measured and recorded over the 10 minute duration. The last background current reading provides the baseline background current for each 10 minute cycle. The transient background current is calculated by subtracting the baseline background current from the measured background current. Integration of the transient background current over the 10 minute period provides the total background signal due to the biosensor. After 3 hours of measurements at 32xc2x0 C., the temperature is raised to 37xc2x0 C., and after the temperature is stabilized the transient background current at 37xc2x0 C. is measured and the total background signal calculated. A total background signal at 32xc2x0 C. of  less than 35,000 nano-Coulomb (nC) is acceptable. An increase in background signal of  less than 15%/xc2x0 C. is acceptable.
The present invention will be described in further detail by giving practical examples. The scope of the present invention, however, is not limited in any way by these practical examples.